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FEC 2004 Measurements and Modeling of Plasma Flow Damping in the HSX Stellarator S.P. Gerhardt, A.F. Almagri, D.T. Anderson, F.S.B. Anderson, D. Brower, J.M. Canik, C. Deng, W. Guttenfelder, K.M. Likin, V. Sakaguchi, J.N. Talmadge, and K. Zhai

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FEC 2004 Outline Description of experiments and diagnostics Studies of flow and electric field evolution –Asymmetries between the spin-up and relaxation –Two time-scale flow evolution –Reduced damping with quasisymmetry Neoclassical modeling of flow damping –Original model for the spin-up Measurements/modeling comparison –Reduced flow damping in quasisymmetric configurations –Flow damping larger than the neoclassical prediction R 1.3m B=0.5T P ECH <200 kW @ 28 GHz HSX is located at the University of Wisconsin- Madison

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FEC 2004 HSX Provides Access to Configurations With and Without Symmetry QHS: Helical Bands of Constant |B| QHS Configuration Mirror Configuration Mirror: Helical Bands are Broken Red |B| 0.5 T Blue |B|<0.5 T

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FEC 2004 Probes and Electrodes Used to Study Flow Damping Bias Electrode to Drive Flows Multi-Tipped Mach Probes Simultaneously Measure Toroidal and Poloidal Flows 16 channel H array to determine the neutral density

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FEC 2004 Biased Electrode Experiments Demonstrate New Flow Phenomena: 1)Reduced Flow Damping with Quasisymmetry 2)Two Time-Scale Flow Evolution

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FEC 2004 Time (msec.) Preview: QHS Flows Damp More Slowly, Goes Faster For Less Drive QHS: 8 A of electrode current Mirror: 10 A of electrode current QHS Mirror All other parameters (n e =1x10 12 cm -3, n n 1x10 10 cm -3 T i 25eV, B=0.5T, P ECH =50 kW) held constant.

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FEC 2004 Asymmetries and Multiple Time- Scales Observed in Flow Evolution Potentials: Fast Rise and Slow Decay Electrode Current: Large Spike and Fast Termination Plasma Flows: Fast and Slow Time- Scales at Rise and Decay

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FEC 2004 Neoclassical Modeling Goal: Assess the flow damping caused by 1)Symmetry breaking ripples 2)Ion-neutral friction

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FEC 2004 Solve the Momentum Equations on a Flux Surface Two time-scales/directions come from the coupled momentum equations on a surface Use Hamada coordinates, linear neoclassical viscosities, neglect heat fluxes Steady state solution yields radial conductivity

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FEC 2004 Spin-Up and Spin-Down are Treated Differently in Modeling At bias turn-on, switches put voltage on the electrode (~1 sec.). Measurements show electric field is established on the electrode voltage-rise time-scale. Spin-Up Model: Flows and radial current respond to the electrode potential rise. At bias turn-off, switches break the electrode current (~1 sec.). Relaxation Model: Flows and electric field respond to the electrode current termination.

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FEC 2004 Flow Rise: Electric Field is Turned on Quickly Assume that the electric field, d /d , is turned on quickly ExB flows and compensating Pfirsch-Schlueter flow grow on the electric field time-scale Parallel flow grows at a “Hybrid rate” F determined by viscosity and ion-neutral friction Two time-scales/two direction flow evolution Toroidal Damping Poloidal Damping

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FEC 2004 Flow Decay: External Radial Current is Quickly Turned Off f ( ) (fast), and s ( ) (slow rate) are flux surface quantities related to the geometry and ion-neutral collision frequency. Break the flow into parts damped on each time-scale: Large neutral density (n n =1x10 12 cm -3 ) in this calculation. Slow rate corresponds to flows in the direction of symmetry. Numerically calculated Hamada basis vectors used in this figure. B Symmetry Fast Slow Total Flow +

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FEC 2004 The Hybrid Rate is Intermediate to the Fast and Slow Rate Fast Rate is faster than Hybrid Rate, F is faster than Slow Rate

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FEC 2004 Mirror Shows Increased Neoclassical Damping Compared to QHS Fast rates are comparable Mirror F is larger by a factor of 2-3 Mirror slow rate is larger by 1-2 orders of magnitude QHS/Mirror Comparison

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FEC 2004 Comparison of Neoclassical Theory with Measurements 1) Reduced Flow Damping with Quasisymmetry 2) Evidence of Anomalous Flow Damping

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FEC 2004 QHS Radial Conductivity is Larger than the Neoclassical Prediction

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FEC 2004 Modeling Predicts the Difference in the QHS and Mirror Slow Rise Rates Mirror flows rise more quickly than QHS. Neoclassical hybrid time F shows good agreement with the measurements. Flow Rise Rate

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FEC 2004 Flow Decay Rates Show Reduced Damping with Quasisymmetry Conclusion Quasisymmetry reduces flow damping, even in the presence of some anomalous damping. Neoclassical model predicts a much slower decay than the measurements (Factor of 10 in QHS, factor of 3-5 in Mirror). Difference between measurements is comparable to the difference between the models. Slow Flow Decay Rate

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FEC 2004 Summary We have observed 2 time-scale flow evolution in HSX. An original model for the spin-up reproduces many of the features in the measurement. The damping in the symmetry direction appears to be larger than the neoclassical prediction with neutrals. The QHS configuration exhibits reduced damping compared to a configuration with the symmetry broken.

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FEC 2004 The End

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FEC 2004 Similar Flow Rise Rates Simultaneously Measured at High and Low Field Locations All relevant time-scales are similar on high and low field sides Slow Flow Rise Time Floating Potential Decay Time Fast Flow Decay Time Slow Flow Decay Time Floating Potential and J sat profiles are similar at both locations as well. r/a Flow Rise Rate

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FEC 2004 Two Time-Scale Model Fits Flow Evolution Similar time-scales measured by LFS and HFS probes. Rise and Fall Not Symmetric U 1 =M(t)cos( f (t)) U || U 2 =M(t)sin( f (t)) U

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FEC 2004 Both Flow Speed and Direction Evolve over the Electrode Pulse Bias Pulse Duration Need to extract the time-scales and directions. (msec.)

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FEC 2004 Voltage Application Initiates the Rise, Current Termination Initiates the Decay

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FEC 2004 Developed a Comprehensive Set of H α Detectors for Neutral Density Measurements Toroidal array: 7 detectors on magnetically equivalent ports Poloidal array: 9 detectors Gas Puff Here All detectors absolutely calibrated Analysis done by J. Canik using DEGAS code

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FEC 2004 Mach Probes Used to Measure Time- Dependent Plasma Flows 6 tip mach probes measure plasma flow speed and direction on a magnetic surface. 2 similar probes are used to simultaneously measure the flow at high and low field locations, both on the outboard side of the torus. Data is analyzed using the unmagnetized model by Hutchinson. Time response of ~10-20 s Probe measures V f with a proud pin. Looking To The Magnetic Surface

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FEC 2004 We Have Developed a Method to Calculate the Hamada Basis Vectors Method involves calculating the lab frame components of the contravariant basis vectors along a field line, similar to that by V.V. Nemov. Need initial condition on the basis vectors to complete this integration. Knowing at outboard symmetry plane is sufficient for calculating the initial conditions. Use two methods of computing the Pfirsch-Schlueter current to derive initial condition... Method by Nemov 1, h is numerically calculated Method by Coronado and Wobig 2, B is the desired quantity Poloidal Basis Vector Toroidal Basis Vector Radial Basis Vector 1) V.V. Nemov, Nuclear Fusion 30, 927 (1990), 2) M. Coronado and H. Wobig Phys Fluids B 4, 1294 (1992)

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FEC 2004 Floating Potential is a Flux Surface Quantity High Field Side Low Field Side

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FEC 2004 Electrode Characteristics at Turn Off Fit the Decay Model Electrode Current Turns off in ~1 s Electrode Voltage Decays in ~30-50 s Floating potential and fast component of flow decay on same time-scale as electrode voltage, in agreement with neoclassical fast rate.

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FEC 2004 Artificially Increasing the Damping Improves Theory/Experiment Comparison Increase the neutral density to simulate extra damping. This agreement comes at the cost of the rise model agreement. Need a better model for the enhanced damping. Steady State Bias Induced Flows Agree Better Predicted Measured Symmetry B Radial Conductivity Agrees Better

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FEC 2004 Steady State Flow Direction Differs Somewhat from Neoclassical Prediction (n,m)=(4,1) symmetry direction Magnetic Field Predicted Flow Direction Measured Flow Direction This sort of comparison is only possible if the basis vectors are known: U=U e + U e

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FEC 2004 Neoclassical Theory, Including Neutrals, is a Candidate to Explain Flow Damping in HSX Near the edge, there are a number of growing symmetry breaking terms in the Hamada spectrum. Low density plasma allows significant neutral penetration.

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FEC 2004 Synthesis of These Comparisons Measured fast time-scales match the neoclassical predictions. Slow time-scale is significantly faster than the neoclassical prediction. Appears that the damping in the direction of symmetry is faster than neoclassical. Large tokamaks have usually seen anomalous toroidal flow damping (DITE, ISX-B, PLT,PDX, ASDEX, TFTR, DIII-D, JET, C-MOD…) Smaller tokamak biased electrode experiments show anomalously large radial conductivity (barring neutrals, any radial current is anomalous!) HSX is quite similar to the tokamak results in this sense.

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FEC 2004 The End

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